This invention relates to diagnostic instruments for facilitating the maintenance and repair of vehicle transmissions and more particularly to an instrument for facilitating the detection and location of malfunctions in a pneumatic circuit which controls a transmission in response to movements of an operator's control lever or the like.
Prior U.S. Pat. No. 3,508,540 discloses a form of semi-automatic transmission for vehicles or the like in which clutching, braking and gear shifting operations required for effecting a change of speed ratio or drive direction or both are all accomplished and sequenced by pneumatically operated devices in response to movement of an operator's control lever. In contrast to older forms of mechanical transmission of the kind having sets of ratio gears, the operator need not actuate and modulate a clutch and need not supply the mechanical force needed for clutching and gear shifting operations. While this greatly simplifies the tasks of the operator, a complex pneumatic circuit is required in order to realize this result. Prior U.S. Pat. Nos. 3,570,636; 3,667,309 and copending application Ser. No. 452,116 of Philip S. Webber, filed Mar. 18, 1974 for CONTROL SYSTEM FOR SHIFTING A DRIVE TRANSMISSION, assigned to the Assignee of the present application, disclosed several pneumatic circuits suitable for this purpose.
As may be seen by reference to the above-identified prior U.S. Pat. No. 3,667,309, effective operation of the transmission is dependent on the presence and the absence of fluid pressures in a large number of control circuit flow paths at particular times during operation of the system. Heretofore, testing of such a control circuit in the field to assure that pressure signals are present or absent from key points in the circuit as necessary at various phases of operation has been a very difficult and tedious task. If a simple pressure gage is used for this purpose, it must repeatedly be connected and disconnected from a large number of points in the circuit. Repeated partial disassembly and reassembly of circuit components may be necessary and a variety of special fittings may be needed. In some cases, the pressure signals to be detected are very brief and many pressure gages do not respond fast enough to provide a reliable indication of such signals. The difficulty of performing such tests in the field creates a problem from the standpoint of conducting routine maintenance checks to assure that the transmission control system is in good order and also complicates the task of identifying the particular source of a malfunction when one is known to be present.